This invention relates generally to guided missiles, and more particularly to a method of terminal guidance of such a missile.
One known method for determining the position of the target is based on processing signals from a TV or infrared (IR) imaging system to derive the requisite guidance commands. Ordinarily the signals out of an IR or TV sensor are converted to an array of digital words (sometimes hereinafter called "pixels") with the value of each word representing the intensity of IR energy radiating from a different point within a field of view. Electronic circuitry then is used to process the array to select any cluster of pixels that is known, a priori, to correspond with a cluster indicative of a target. Further processing of a selected cluster in any conventional fashion finally produces the requisite guidance commands. These commands are usually based on features of the target including its edges, which define its shape and angular size as seen by the imaging system. Successive frames from the imaging system are processed with the guidance commands generated for each frame, to guide the missile until it intercepts its target.
A problem arises in guiding the missile as it nears its target. The field of view of the IR sensor is generally very narrow. As the missile approaches the target, the target fills more and more of the field of view, creating an effect similar to what is observed when a camera is "zoomed" in for a closeup (sometimes hereinafter called "growth" of the image). At some time during the approach, the target fills the entire field of view. From that time on, the missile is deemed to be in the "terminal phase". During the terminal phase, the features used to generate guidance commands, particularly the edges of the target, may disappear. As the features disappear, the guidance commands may become indeterminate. Alternatively, the system may guide the missile towards an edge which stays in the field of view, and some percentage of missiles will miss the target.
Even in the terminal phase it may be necessary to make course corrections to guide the missile toward the target. In some instances, the field of view of the sensor may be so narrow or the target so large that the missile is an appreciable distance from the target when the target fills the field of view. Without guidance, the missile could drift appreciably off its desired course as it traveled that distance and might miss the target entirely. Alternatively, the target might be so large that the missile must strike a particular aim point in the target to be effective. In such instances, course corrections are needed during the terminal phase of the missile flight to guide it toward the aim point.
One known guidance technique which does not depend upon particular features of the target being within the field of view is correlation tracking. In correlation tracking, a stored scene is compared with the scene from the imaging system. The amount and direction the stored scene must be moved to best match the scene from the imaging system determines the magnitude and direction of the guidance command.
As the missile enters the terminal phase, the image of the target is stored as a reference. The images in successive frames from the imaging systems are then compared with this reference scene to derive the guidance information. Thus, the aim point contained in the reference scene is preserved.
Since the image continues to grow as the range to target decreases, the stored image, which is not growing, will soon not correlate with images in the successive frames. At this point, a new reference image must be exchanged for the stored image. Exchanging reference images continues at an ever increasing rate until target impact.
Every time the stored reference image is exchanged, it incorporates whatever error is present. For example, error is introduced if the first image is exchanged for an image representing a portion of the target slightly offset from the portion of the target represented by the center of the first image. Exchanging images therefore results in noise and drift in the guidance command. The more often the reference scene is exchanged, the larger the drift in guidance command will become. This drift will result in the missile missing its original aim point, and the greater the drift, the greater the miss.
If one considers an incremental area on the target offset from the aim point of the missile and within the field of view, two phenomena are present as one observes the image received by the missile guidance system as the aim point is approached. First, points in the image of the incremental area move out radially from the aim point at a rate proportional to both the velocity of the missile and the distance between the aim point and the incremental area. Second, the portion of the image representing that incremental area on the target will grow in an angular size as seen by the sensor as the range decreases. To reduce the number of required reference scene updates, a correlation tracker must address these phenomenon.
In a known variation of a correlation tracker, a cluster of pixels in the image made of the target as it enters the terminal phase is selected as a reference cluster. That cluster is divided into a predetermined number of subclusters. As successive frames are produced, the reference subclusters are independently matched to clusters in the frames. Guidance commands are generated based on the amount and direction each of the subclusters must be moved to best match a portion of the image in a successively generated frame.
The foregoing approach compensates partially for changes between the successive images by allowing the subclusters to be matched to areas that have moved radially outward from the center of the image. It would be desirable to provide an approach which also compensates for growth of the subclusters. It would also be desirable to provide an approach which is computationally simple.